The area has been evacuated as security agencies carry out their work.
According to one employee alert the vehicle is located on the Unit Block of First Street SE.
A large number of police officers, and other law enforcement personnel, are on the scene.
CNN reporter Jim Sciutto said officers are “sending negotiators to engage with a man in the truck making the threat”.
The FBI also has officers on the scene, as confirmed by its Washington Field Twitter account.
It said: “The FBI Washington Field Office’s National Capital Response Squad is responding with our partners, including Capitol Police, to reports of a suspicious vehicle and bomb threat near the Library of Congress.”
US Capital Police tweeted they are “responding to a suspicious vehicle near the Library of Congress”.
They added: “Please stay away from this area and follow this account for the latest information.
“This is an ongoing investigation. We are monitoring this situation closely and will update this account as we get information we can release.”
Nathaniel Reed, a Washington DC based journalist, said those inside the Capital building have been asked to take cover.
He said: “Capital Police is now issuing an internal security threat directing some on the Capital Complex to:
“Close, lock and stay away from external doors and windows.
“Find a place to hide or seek cover if in a public space.
“Remain quiet and silence electronics.”
Earlier he tweeted: “One House Office Building and one Library of Congress building have been evacuated.
“A second Library of Congress building is under a shelter in place order.”
The Library of Congress is situated near both the Capitol building and US Supreme Court.
Congress is on break this week, with many lawmakers in their home states.
Speaking on condition of anonymity, law enforcement officers said they are trying to establish whether it is a viable explosive device.
On January 5 pipe bombs were left outside the headquarters of both the Democratic National Committee, and the Republican National Committee, in the capital.
Neither exploded and there were no reports of any injuries.
The following day Congress was stormed by thousands of pro-Trump rioters, angry at the November 2020 presidential election result.
One rioter was shot dead by law enforcement inside the Congress building.
More to follow…